In recent years, the use of the pultrusion process to form composite structure has become increasingly popular since it enables such structures to be fabricated on a continuous basis. As the process is commonly practiced, the reinforcing material, for example, glass filaments, or other reinforcing fibers such as carbon and high strength organic fibers, combined in associated groupings or "tows" are passed through a tank containing the polymer which is to form the continuous phase in the form of a liquid solution, or in melted form. The plastic coated tows are thereafter drawn through a heated die, the coated tows emerging therefrom as a relatively rigid composite. In the case of pultrusions which employ thermoplastic polymers, the pultruded products can thereafter be subdivided into short segments, commonly known as "prepreg", which can be injection or compression molded to form articles of superior strength.
In carrying out the process according to the procedures previously known, however, and with reference to the use of thermosetting polymers as the continuous phase, the polymers must undergo a time-consuming condensation reaction which results in a high degree of polymerization shrinkage, in addition to the normal thermal shrinkage. Consequently, in order to avoid cracking of the composite caused by shrinkage, the cure cycle must be undesirably slow and carefully controlled.
When resort is to thermoplastic polymers, on the other hand, polymers with advantageously high glass transition temperatures, t.sub.g, also have undesirably high viscosities, even at elevated temperatures. Consequently, in order to reduce their viscosities so that "wetting" of the reinforcing filaments can take place to the degree necessary, it is desirable to heat the materials to as high a temperature as possible, but below their decomposition point. Unfortunately, however, the difference between the temperature at which practical viscosities are experienced, and that at which polymer decomposition commences is frequently relatively small, making control of the process difficult.
The problem of filament wetting has previously been recognized, and various methods have been proposed to avoid it, for example, as shown in European Patent Application Publication Nos. 0 056 703, and 0 102 159. Such methods have not been altogether successful, however, at least to the fact that they present other disadvantages, for instance, in some cases a need to melt the polymers. In instances where thermoplastic polymers must be melted to carry out the required coating of the filaments, the polymerization, grinding, and melting steps preparatory to the coating operation involving processing and energy expenditure which are costly, and thus desirable to avoid if possible. Furthermore, maintaining a polymer in a melting condition over protracted periods inevitably leads to the gradual degradation of the polymer, detrimentally influencing its physical properties.
In addition, and irrespective of the nature of the system used to coat the filaments during the pultrusion process, it is frequently difficult to adequately wet the filaments, due to the relatively high viscosities of the coating liquid. Uncoated, or improperly coated filaments are prone to abrasion damage in subsequent handling and processing, leading to broken and, therefore, shortened filaments which provide poor physical properties in the composite, particularly in making prepreg for injection molded products. Consequently, poor wetting tendencies must be offset either by lowering the processing speeds, resulting in increased cost, or undesirably reducing reinforcing material loadings, detrimentally influencing physical properties, or both.